Radio Broadcast (Nov 1926-Apr 1927)

502 RADIO BROADCAST ADVERTISER Never BeforeSuch Tone Quality Sterling perfected *rB" Eliminator control gives this added Advantage PERHAPS you regard a "B" Eliminator merely as a means of doing away with the expense and nuisance of forever replacing worn -down "B" Batteries. But with the Sterling "B" Eliminator, there is a tremendous improvement in tone quality too! The secret of this amazing tone quality lies in the extremely fine control of the "B" voltage. This definite Sterling advantage in tone quality is possible only by using first quality materials, high standards of manufacture, backed by 20 years of Sterling electrical experience. This Sterling Dependable Light Socket Power Unit RT-41 is the ideal "B" Power supply for sets using as many as 5 tubes. It takes up no more room than a 45 volt dry battery — yet it gives three times the voltage — 135 volts. Just right for Radiolas 25 and 28. Price complete with ^TC C\C\ CX 313 tube . . ^O.VU Also Raytheon Tube Types for high-powered sets R-97— "B" and "C" Power • $55.00 R-99— "B" Power .... $45.00 For every set — there's a "B" ELIMINATOR Send for booklet " J " telling about the complete line of Sterling Radio Equipment. The Sterling Mfg. Co. 2831 Prospect Ave. •• Cleveland . 75 Radio Broadcast Laboratory Information Sheet Interference Finder March, 1927 A PORTABLE RECEIVER ON LABORATORY Sheet No. 76 is given a circuit diagram of a small portable receiver for use in locating sources of interference. In order to make up this receiver, the following apparatus is necessary: L — Any standard loop, tapped at the center. Ci — Variable condenser designed for operation with the loop that is used. Any value between 0.00025 mfd.and 0.0005 mfd. is satisfactory. Rg — 4-megohm grid leak. C2 — 0.00025-mfd. grid condenser. C3 — Midget condenser, 0.00015 mfd. max. Ti, T2 — Audio-frequency transformers. J — Single-circuit filament control jack. R — 20-ohm rheostat. Ll — Radio-frequency choke coil which may consist of 400 turns of No. 32 or smaller wire wound on an ordinary spool. To operate this receiver the following accessory equipment is necessary: Three ordinary dry cells for the filament circuit; one small 45-volt battery for the plate circuits of all three tubes. Three 199 tubes. The receiver is of the ordinary regenerative type. The condenser, O, controls the tuning, while condenser Cj controls the amount of regeneration. When Ca is advanced near to its maximum position the detector tube will oscillate so that stations may be picked up by a heterodyning whistle. The loop should be mounted so that it can be turned in any direction. It will be found that the loop is very directional and that, therefore, in interference investigations, the interference will be picked up loudest when the loop is pointing toward the source. In this way the actual source of interference can often be located. The 1 receiver should be made extremely portable and, for this reason, it is preferable to enclose the batteries and loop, as well as the receiver, in a small carrying [case (that can be easily handled. The receiver should be equipped with a jack into which a separate loop unit may be plugged.or the loop may be wound inside the case itself. The filament rheostat controlling the tubes should not be advanced further than necessary to obtain satisfactory reception since excessive filament voltage on the 199 tube is very detrimental. It will be found that three ordinary dry cells used for filament lighting will last for many months. Radio Broadcast Laboratory Information Sheet March, 1927 Interference Finder No. 77 Radio Broadcast Laboratory Information Sheet Interference Elimination March, 1927 USE OF CONDENSERS AND CHOKES WHEN interference is experienced from motors, telephone ringers, or other similar apparatus, it may frequently be eliminated by using some such circuit arrangement as is illustrated on this Sheet. This circuit is called a filter, and can easily be connected to practically any piece of apparatus that is causing interference. If, for example, a motor is found to be causing the interference, such may be eliminated by connecting two condensers across the line terminals of the motor with the mid point grounded, as illustrated in the sketch. The values of the condensers should, in general, not be less than 2 mfd., although smaller size condensers will sometimes give satisfactory results. In extreme cases of interference, where it is found that the condensers shunted across the line with their mid point grounded do not remedy the trouble, the inclusion of choke coils in series with the line is necessary. These choke coils must be fairly large and wound with wire large enough to carry the full current in the line. The shunting condensers and the choke coils, if they are used, should always be placed as close to the source of interference as possible. The source of interference is not always obvious, and, for that reason, it is advisable first to make some simple tests to make certain of the cause. In such an investigation the small portable receiver described on the Laboratory Sheets Nos. 75 and 76 will be found very useful. Before installing any condensers, one should make certain that they have a rating sufficiently high enough to withstand the voltages under which they Powei SuppK Source 01 Interference Motor. Telephone Ringer.etc must operate. There will be little difficulty in obtaining satisfactory condensers for use on directcurrent circuits since there are many on the market rated at as high as 1000 volts d.c. Frequently these same condensers are not satisfactory for use on a. c. circuits, however, and consequently, if the device to be shunted is operated from a.c, make sure that the condensers used have a satisfactory a.c. rating.